Make the popup menu width animation work

sky/sdk/example/stocks/lib/stock_list.dart

Index: sky/sdk/example/stocks/lib/stock_list.dart
diff --git a/sky/sdk/example/stocks/lib/stock_list.dart b/sky/sdk/example/stocks/lib/stock_list.dart
index 2cfd2ede4cb28732e9cfc839eb63fd8f56590e68..8316de683c1c6eaaa64268b8a5f6e9b7a8041abe 100644
--- a/sky/sdk/example/stocks/lib/stock_list.dart
+++ b/sky/sdk/example/stocks/lib/stock_list.dart
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:sky/widgets/scrollable_list.dart';
+import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/basic.dart';
 
 import 'stock_data.dart';
@@ -14,10 +15,13 @@ class Stocklist extends Component {
   final List<Stock> stocks;
 
   Widget build() {
-    return new ScrollableList<Stock>(
-      items: stocks,
-      itemHeight: StockRow.kHeight,
-      itemBuilder: (stock) => new StockRow(stock: stock)
+    return new Material(
+      type: MaterialType.canvas,
+      child: new ScrollableList<Stock>(
+        items: stocks,
+        itemHeight: StockRow.kHeight,
+        itemBuilder: (stock) => new StockRow(stock: stock)
+      )
     );
   }
 }